How did this footage retrieved if NASA had never gone back to the Moon?

There is only one explanation, an explanation that will save this movie from every plothole.

Here it is: This movie was not the actual footage from any Apollo 18 mission, Apollo 18 never happened as the government claims. The movie was just a home made project of some bored guy/girl who only wanted to stir things up through conspiracy nuts. similar to those "UFO sighting" videos. That's why there are so many loop holes in the movie like ambient sound and background noises on moon, gravity similar to earth, astronomer not acting like astronomers etc etc.

someone put together a bunch of guys, probably his/her friends and cousins, bought a couple of Halloween costumes, shot the movie from whatever camera s/he had available, and edited it on his/her own/borrowed macbook and voila, you had this movie. The authenticity of the claims and the footage itself is never established in the movie.

This is the only explanation I can come up with.

Smart kids!
I guess you also found out about the minor plot hole in Superman kind of movies: people actually don't fly!

This will all be explained in the next one: Apollo 19.